1. Field of the Invention
The invention relates to a tunneling microscope, more particularly to a radio-frequency reflectometry scanning tunneling microscope.
2. Description of the Related Art
Referring to FIG. 1 and FIG. 2, two schematic diagrams are provided for illustrating different scanning operations of a conventional scanning tunneling microscope (referred to as a tunneling microscope hereinafter for the sake of brevity). Operating principles of the tunneling microscope reside in that a bias voltage (i.e., a voltage difference) is applied between a probe 10 and an object 20 to be scanned. When the probe 10 is disposed sufficiently close to a surface of the object 20, the probe 10 is adapted to allow a tunneling current It to flow between the probe 10 and the object 20 as a result of the tunneling effect. A larger magnitude of the tunneling current It usually indicates a closer distance between the probe 10 and the object 20 (i.e., a tunneling distance). When the probe 10 is moved away from the object 20, the magnitude of the tunneling current It decreases exponentially. The tunneling microscope may obtain a result associated with the surface of the object 20 based on the tunneling current It during a scanning process, as shown in FIG. 1 and FIG. 2.
There are two kinds of the scanning operations of the tunneling microscope. The first one is a constant height mode which is illustrated in FIG. 1. In this mode, the tunneling microscope may depict the surface of the object 20 based on the magnitude of the tunneling current It. Owing to an exponential relationship between the tunneling current It and the tunneling distance, the constant height mode is usually adopted for rapid small area scans, so as to prevent the probe 10 from colliding with the object 20. The second kind of the scanning operations is a constant current mode which is illustrated in FIG. 2. In this mode, the tunneling current It obtained during the scanning process is fed back to a scanning control circuit 12 shown in FIG. 3, which adjusts an altitude of the probe 10 relative to the object 20 along a Z-axis based on the tunneling current It, such that the tunneling current It is maintained at a constant value. By analyzing a track of the probe 10 along the Z-axis in combination with X-Y plane scans, a high resolution scanning result of the surface of the object 20 may be obtained.
Referring to FIG. 3, a current-to-voltage converter 11 adopted in the conventional tunneling microscope is illustrated. The current-to-voltage converter 11 amplifies and converts the tunneling current It into a voltage Vout, and outputs the voltage Vout to the scanning control circuit 12. The scanning control circuit 12 controls the altitude of the probe 10 based on the voltage Vout. A gain of the current-to-voltage converter 11 is decided by a feedback resistor RFB therein. Since the amplitude of the tunneling current It is very small (pico-amp to nano-amp scale), in practice, the feedback resistor RFB is set at 100M Ohms or larger. Moreover, since an intrinsic capacitor CFB of the current-to-voltage converter 11 is usually of picofarad scale, a time constant RFB×CFB attributed to the feedback resistor RFB and the intrinsic capacitor CFB limits a frequency of the voltage Vout to about tens of kilohertz, such that the current-to-voltage converter 11 is limited to operate at no more than hundreds of kilohertz. This limitation causes the conventional tunneling microscope to be suitable only for observing physical and chemical phenomena that are static or dynamic but with relatively slow evolution.
Accordingly, in order to overcome the aforementioned issue that the operating bandwidth of the tunneling microscope is limited, a radio-frequency (RF) scanning tunneling microscope has been proposed by Kemiktarak et al., and a circuit diagram thereof is illustrated in FIG. 4. Referring to FIG. 4, a right-hand side of a circuit of the RF scanning tunneling microscope shows that a similar circuit structure of the conventional tunneling microscope is adopted. That is to say, the tunneling current It generated between the probe 10 and the object 20 is outputted, via an inductor L having one end coupled electrically to the probe 10 and via a bias tee circuit 13 coupled electrically to another end of the inductor L, to the current-to-voltage converter 11. The current-to-voltage converter 11 amplifies and converts the tunneling current It, and then outputs the tunneling current It thus amplified and converted to the scanning control circuit 12, so as to control the altitude of the probe 10. The scanning control circuit 12 further depicts the surface of the object 20 based on the tunneling current It outputted by the current-to-voltage converter 11. In addition, a left-hand side of the circuit of the RF scanning tunneling microscope is an RF scanning circuit. A grounding capacitor C is coupled electrically to said one end of the inductor L. The RF scanning circuit includes an RF source 14 outputting an RF signal via a directional coupler 15, via the bias tee circuit 13 that has two ports passing RF signals bi-directionally and a third port passing DC signal, and via the inductor L to the probe 10. The probe 10 cooperates with the object 20 to form a tunneling resistor Rt therebetween, such that the inductor L is connected in series with a parallel connection of the tunneling resistor Rt and the capacitor C, so as to form an L-leg low pass LCR resonant circuit. An equivalent circuit of the L-leg low pass LCR resonant circuit is illustrated in FIG. 5, and a resonant frequency thereof is defined by
            ω      LC        =                            1          LC                ⁢                  (                      1            -                          L                              CR                t                2                                              )                      ,where ωLC represents an angular frequency associated with the resonant frequency, L represents inductance of the inductor L, C represents capacitance of the capacitor C, and Rt represents resistance of the tunneling resistor Rt, i.e., a tunneling barrier.
It is evident from the abovementioned function that the resonant frequency is substantially determined after the inductance of the inductor L and the capacitance of the capacitor C have been decided. Resistance of the tunneling resistor Rt is adjusted according to
            R      t        ≈          L              C        ×                  Z          o                      ,where Zo represents output impedance of the RF scanning circuit that outputs the RF signal to the L-leg low pass LCR resonant circuit, such that impedance Zt of the L-leg low pass LCR resonant circuit corresponds to the output impedance Zo, so as to achieve optimum impedance matching. At this time, the L-leg low pass LCR resonant circuit may obtain substantially the maximum power of the RF signal (i.e., maximum power transfer).
Referring to FIG. 6, when the resistance of the tunneling resistor Rt is adjusted to 0.15M Ohms, a relatively good impedance matching may be achieved, and return loss is 45 dB. If the probe 10 is moved away from the object 20 along the Z-axis, resistance of the tunneling resistor Rt increases. At the same time, since the impedance Zt of the L-leg low pass LCR resonant circuit does not match with the output impedance Zo, power of a reflected RF signal resulting from reflection of the RF signal by the L-leg low pass LCR resonant circuit also increases. FIG. 6 illustrates the phenomena where a reflection coefficient for the L-leg low pass LCR resonant circuit increases rapidly (i.e., the return loss decreases) while the resistance of the tunneling resistor Rt increases. Further, it is evident from FIG. 6 that when the resistance of the tunneling resistor Rt changes from 0.15M ohms to 0.4M ohms, the return loss changes from 45 db to 15 dB. When the resistance of the tunneling resistor Rt is equal to 1M ohms, the return loss is 12 dB. When the resistance of the tunneling resistor Rt ranges from 10M ohms to 1 G ohms, the return loss is kept at around 11 dB, and variation of the return loss is so subtle that, in practice, it is hard to distinguish the differences between the return loss while the resistance of the tunneling resistor Rt is set at around 10M ohms and at around 100M ohms. Moreover, in practical applications, the resistance of the tunneling resistor Rt usually ranges from 10M ohms to 1 G ohms, so that the return loss is hardly utilized for obtaining actual resistance of the tunneling resistor Rt by means of the aforementioned RF scanning tunneling microscope.
It is evident from FIG. 7 that a relationship between the return loss and the resistance of the tunneling resistor Rt is a non-monotonic function. That is, the same return loss may correspond to two different resistances of the tunneling resistor Rt. The non-monotonic relationship may cause misjudgment of the scanning result made by the scanning control circuit 12, and instability may arise when control of the altitude of the probe 10 is based on the return loss.